Cellulose esters such as cellulose triacetate (CTA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB), are used in a wide variety of films by the liquid crystal display (LCD) industry. Most notable is their use as protective and compensator films in conjunction with polarizer sheets. These films are typically made by solvent casting, and then are laminated to either side of an oriented, iodinated polyvinyl alcohol (PVOH) polarizing film to protect the PVOH layer against scratching and moisture ingress, while also increasing structural rigidity. Alternatively, as in the case of compensator films, they can be laminated with the polarizer stack or otherwise included between the polarizer and liquid crystal layers. Cellulose esters have many performance advantages over other materials that see use in display films such as cycloolefins, polycarbonates, polyimides, etc. However, optical birefringence requirements currently often dictate that the latter materials be used instead.
In addition to serving a protective role, these films also play a role in improving the contrast ratio, wide viewing angle, and color shift performance of the LCD. For a typical set of crossed polarizers used in an LCD, there is significant light leakage along the diagonals (leading to a poor contrast ratio), particularly as the viewing angle is increased. It is known that various combinations of optical films can be used to correct or “compensate” for this light leakage. These films must have certain well-defined birefringences (or retardations) that vary depending on the type of liquid crystal cell used, since the liquid crystal cell itself will also impart a certain degree of undesirable optical retardation that must be corrected. Some of these compensator films are easier to make than others, so compromises are often made between performance and cost. Also, while most of the compensator and protective films are made by solvent casting, there is a push to make more films by melt extrusion.
Compensator and optical films are commonly quantified in terms of birefringence which is related to the refractive index n. The refractive index is typically in the range of 1.4 to 1.8 for polymers in general, and approximately 1.46 to 1.50 for cellulose esters. For a given material, the higher the refractive index, the slower the speed of light propagating through it.
For an unoriented isotropic material, the refractive index will be the same regardless of the polarization state of the entering light wave. As the material becomes oriented, or otherwise anisotropic, the refractive index becomes dependent on material direction. For purposes of the present invention, there are three refractive indices of interest denoted as nx, ny, and nz, which correspond to the machine direction (MD), the transverse direction (TD), and the thickness direction, respectively. As the material becomes more anisotropic (e.g., by stretching it), the difference between any two refractive indices will increase. This difference is referred to as the “birefringence.”
Because there are many combinations of material directions to choose from, there are correspondingly different values of birefringence. The two that are the most common, namely the planar birefringence Δe and the thickness birefringence Δth, are defined as:Δe=nx−ny  (1a)Δth=nz−(nx+ny)/2  (1b)
The birefringence Δe is a measure of the relative in-plane orientation between the MD and TD directions and is dimensionless. In contrast, Δth gives a measure of the orientation of the thickness direction, relative to the average planar orientation.
Another term often used to characterize optical films is the optical retardation (R). R is simply the birefringence times the thickness (d) of the film in question. Thus,Re=Δed=(nx−ny)d  (2a)Rth=Δthd=[nz−(nx+ny)/2]d  (2b)
Retardation is a direct measure of the relative phase shift between the two orthogonal optical waves and is typically reported in units of nanometers (nm). Note that the definition of Rth varies with some authors particularly with regard to the +/− sign.
The birefringence/retardation behavior of materials is also known to vary. For example, most materials when stretched, will exhibit a higher refractive index along the stretch direction and a lower refractive index perpendicular to the stretch direction. This follows because, on a molecular level, the refractive index is typically higher along the polymer chain's axis and lower perpendicular to the chain. These materials are commonly termed “positively birefringent” and represent most standard polymers including all commercial cellulose esters.
Another useful parameter is the “intrinsic birefringence,” which is a property of the material and is a measure of the birefringence that would occur if the material were fully stretched with all chains perfectly aligned in one direction.
There are two other much rarer classes of materials, namely “negative birefringent” and “zero birefringent.” Negative birefringent polymers exhibit a higher refractive index perpendicular to the stretch direction (relative to the parallel direction), and consequently also have a negative intrinsic birefringence. Certain styrenics and acrylics are known to have negative birefringent behavior due to their rather bulky side groups. Zero birefringence, in contrast, is a special case and represents materials that show no birefringence with stretching and thus have a zero intrinsic birefringence. Such materials are ideal for optical applications as they can be molded, stretched, or otherwise stressed during processing without showing any optical retardation or distortion. Such materials are also extremely rare.
The actual compensator films that are used in an LCD can take on a variety of forms including biaxial films where all three refractive indices differ and two optical axes exist, and uniaxial films having only one optical axis where two of the three refractive indices are the same. There are also other classes of compensator films where the optical axes twist or tilt through the thickness of the film (e.g., discotic films), but these are of lesser importance to understanding the present invention. The important point is that the type of compensator film that can be made is limited by the birefringence characteristics of the polymer (i.e., positive or negative).
In the case of uniaxial films, a film having refractive indices such thatnx>ny=nz  (3a)is denoted as a “+A” plate. In these films, the x direction of the film has a high refractive index while the y and thickness directions are approximately equal in magnitude (and lower than nx). This type of film is also referred to as a positive uniaxial crystal structure with the optic axis along the x-direction. Such films are easy to make by uniaxially stretching a positively birefringent material, using, for example, a film drafter.
In contrast, a “−A” plate uniaxial film is defined asnx<ny=nz  (3b)where the x-axis refractive index is lower than the other directions (which are approximately equal). The most common method for making a −A plate is to stretch a negative birefringent polymer, or alternatively, by coating a negatively birefringent liquid crystal polymer onto a surface such that the molecules are lined up in a preferred direction.
Another class of uniaxial optical film is the C plate, which can also be “+C” or “−C.” The difference between a C plate and an A plate is that in the former, the unique refractive index (or optical axis) is in the thickness direction as opposed to in the plane of the film. Thus,nz>ny=nx(“+C” plate)  (4a)nz<ny=nx(“−C” plate)  (4b)
C-plates can be made by biaxial stretching if the relative stretch in the x and y directions is held constant. Alternatively, they can be made by compression forming. Compressing or equibiaxially stretching an initially isotropic, positive intrinsic birefringent material will result in a −C plate since the effective orientation direction is in the plane of the film. Conversely, a +C plate can be made by compressing or equibiaxially stretching an initially isotropic film made with negative intrinsic birefringent material. In the case of biaxial stretching, if the orientation level is not kept the same in the MD and TD directions, then the material is no longer a true C-plate, but instead is a biaxial film with 2 optical axes.
A third, and more common option for producing C-plates takes advantage of the stresses that form during solvent casting of a film. Tensile stresses are created in the plane of the film due to the restraint imposed by the casting belt, which are also equi-biaxial in nature. These tend to align the chains in the plane of the film resulting in −C or +C films for positive and negative intrinsic birefringent materials, respectively. As most cellulose ester films used in displays are solvent cast, and all are essentially positive birefringent, it is apparent that solvent cast cellulose esters normally only produce −C plates. These films can also be uniaxially stretched to produce +A plates (assuming the initial as-cast retardation is very low), but the ability to make +C or −A plates with cellulose esters is extremely limited.
Besides uniaxial plates, it is also possible to use biaxial oriented films. Biaxial films are quantified in a variety of ways including simply listing the 3 refractive indices in the principal directions (along with the direction of these principal axes). Alternatively, biaxial films are often quantified in terms of the parameter Nz, where Nz is defined asNz=(nx−nz)/(nx−ny)  (5)
Nz is a measure of the effective out-of-plane birefringence relative to the in-plane birefringence and is typically chosen to be about 0.5 when the film is used as a compensator film for a pair of crossed polarizers. In the case when Nz=1, this biaxial film converts to either a +A plate or a −A plate. By optimizing the stretching condition, a certain Nz value biaxial film can be obtained from a cellulose based +C plate.
In order for compensator films to properly eliminate light leakage, they must be combined in certain ways depending on the type of liquid crystal cell used. For example, Fundamentals of Liquid Crystal Displays (D. K. Yang and S. T. Wu, Wiley, N.J., 2006, pp 208-237) describes various ways to compensate for IPS (in-plane switching), twisted nematic (TN), and VA (vertical alignment) type cells using combinations of uniaxial plates (biaxial plates are also effective but are more complicated mathematically). In the case of an IPS cell, a +C plate followed by a +A plate is described (also described is +A followed by +C). When sandwiched between the crossed polarizers, these films effectively correct for light leakage. Another type of structure is where a +A plate is used with a −A plate, which gives a more symmetric viewing angle performance than the +A/+C combination. U.S. Pat. No. 5,138,474 cites the use of +A and −A films together as compensators for TN and super twisted nematic (STN) cells where the +A film is made by stretching polycarbonate and the −A plate is made by stretching negative birefringent polystyrene.
VA compensated films are similar, although the liquid crystal layer itself acts as a +C structure, which has to be figured into the calculation (unlike IPS systems where the cell is typically more “neutral”). The end result is that a +A film in conjunction with a −C film is required. This structure can be made solely with positive birefringent materials. However, improved performance is shown if a 3-layer compensator composed of a +A, −A, and −C film, which once again requires a negative birefringent material for the −A layer. Note that the structures described above are just a few of many combinations available and are only meant to illustrate the importance of positive and negative birefringence. Other compensators (e.g., biaxial films and twisted films) are also possibilities that can benefit from having negative intrinsic birefringence.
The Rth values of solvent-cast cellulose triacetate films range from about −20 to −70 nm, but with mixed ester systems, we have observed ranges from about −20 to −300 nm depending on the type of cellulose ester involved (which determines its intrinsic birefringence), the time left on the casting belt (which controls the residual stress in the film), and the type of plasticizers and additives used. Note that by “mixed ester,” we are referring to cellulose esters having more than one ester type such as, for example, cellulose acetate propionate (CAP) or cellulose acetate butyrate (CAB). It is fairly common to add retardation additives or inhibitors to the solvent dope to help raise or lower the as-cast retardation. The amount of retardation in the −C plate can also be enhanced by biaxial stretching or compression. Other films such as a +A plate can be made by subsequent uniaxial stretching, assuming the retardation of the −C film is low initially.
Of note, however, is that −A and +C compensator plates cannot be easily made with cellulose esters because of their positive birefringent nature. Thus, other more costly (or poorer performing) materials have to be used instead.
Currently, commercial films exhibiting +C plate behavior are made using a nematic liquid crystal coating with a subsequent polymerization process. The coating process and liquid crystal material, however, are very expensive and require an additional processing step of coating a film to achieve the desired properties. There are no commercial films exhibiting high +C behavior based on cellulose ester and additives.
Thus, there is a need in the art for films exhibiting +C plate behavior without using a liquid crystal material and without requiring an additional coating step.